Amazon, With Little Fanfare, Emerges as an Advertising Gian…

Amazon.com Inc. handles nearly half of all online sales in the U.S., giving it a popular platform and a wealth of consumer data. Now it’s on track to become the next juggernaut of online advertising, and its rise threatens to upend Silicon Valley’s ad titans and change the way business is done on Madison Avenue.

The online retailer has ascended to the No. 3 spot in the U.S. digital ad market behind the dominant players, Alphabet Inc.’s Google and Facebook Inc. Though Amazon has just 4% of the market now, the company is expanding…